Internal atrial defibrillation is a successful therapy for the treatment of atrial fibrillation(AF). Current internal atrial defibrillators(IAD) use conventional defibrillation technology to deliver high energy shocks with a high likelihood of success. The applied shock strength of 3-10 Joules is above a patient's pain threshold and this limits the broad application of IAD therapy. A technology is proposed that utilizes multiple very low energy (less than .5J) shocks(VLES) with a low probability of first shock success but a high cumulative probability of conversion. A telemetry system will be implanted in an ovine model of AF. The combined animal model and telemetry system will be used as a test bed for VLES atrial cardioversion. The telemetry unit transmits 4 electrograms a distance of 10 meters and delivers continuous and programmed pacing by remote control. This system will be enhanced by adding pressure monitoring and VLES circuitry. It will be used 1) to characterize electrical remodeling and to track changes in infra-atrial pressure induced by the rapid pacing protocol and 2) to measure the energy that corresponds to a 5 percent level of success, and 3) to test three strategies for increasing the VLES probability of success. The three strategies are: 1) pre-VLES overdrive pacing 2) pre-VLES non-linear dynamics based pacing, and 3) use of class III antiarrhythmic drugs with VLES. Epicardial mapping will be used to identify the mechanism of VLES defibrillation and to characterize the effects of the 3 enhancement strategies. The work proposed in this grant will advance the science of remote testing in unanesthetized animals, will characterize the development of electrical modeling and myocardial pathology produced by rapid pacing in sheep, and will test a total of four strategies for very low energy atrial defibrillation. These advances will increase our understanding of the progression of AF and allow a successful therapy for this debilitating disease to be more broadly applied.